Measurement instruments, such as oscilloscopes, may be used to view periodic signals. In some cases, the signals may vary from period to period. For example, in the case of communications signals a periodic carrier signal may be modulated with another signal for conveying information. Periodic signals may also include transient signals, jitter, and/or noise.
An oscilloscope may be used to display waveform images. Waveform images can be produced directly from an analog signal, or from digitized samples of an analog waveform.
Some display devices (e.g., analog oscilloscopes) display waveform images by controlling vertical deflection of a beam of electrons that is being swept horizontally across a phosphor-based screen. As waveforms are traced on the screen, the areas of the phosphorous screen traced by the beam begin to glow. The glow generally persists long enough to enhance visual perceptibility, but fades over time. Areas of the screen that are traced often may glow more brightly than areas that are traced less often. After an area is traced, the glow may begin to fade over time. A user may use this behavior to visualize a collection of waveforms and see the periodic waveform as a bright trace and see modulated and/or noise signals as a region of reduced brightness around the periodic waveform.
Some display devices display images by selective illumination within an array of pixels distributed across the viewable area. Image resolution in a rectangular grid of pixels, for example, is sometimes specified in terms of the spacing between adjacent pixels (e.g., 2400 dots per inch (DPI), 130 pixels per inch (PPI)). Depending on the implementation, the selective illumination of each individual pixel can be controlled according to digital values for black and white, grey scale, or color information.